The present invention relates to a transmitting and receiving apparatus for satellite communications, which uses a random access processing for data transmission.
In order to utilize a microwave band effectively, at a satellite communications system, a random access processing is used. For example, a very small aperture terminal (VSAT) system consists of a hub station, a plurality of VSATs, and a communications satellite. And a packet data communications is performed between the hub station and the plural VSATs via the communications satellite. In this case, the plural VSATs use the same frequency band, and a random access processing is adopted to perform retransmission in case that plural transmitting data collide, with this, the occupied bandwidth is not made to be large.
FIG. 1 is a block diagram showing a structure of a conventional transmitting and receiving apparatus for satellite communications. In FIG. 1, this transmitting and receiving apparatus for satellite communications is a VSAT. And this VSAT provides an antenna 11, such as a parabolic antenna, which radiates and receives radio waves by setting its elevation angle and bearing to the communication satellite, and a transmitter/receiver 12 that transmits a data to the hub station and receives a data from the hub station through the antenna 11. And further this transmitting and receiving apparatus for satellite communications provides a modulator-demodulator (MODEM) 13, which demodulates a signal from the transmitter/receiver 12 and modulates a transmitting data, and outputs the demodulated or modulated data, a random access controller 14 by which microwave radio transmission lines using for the data transmission to and from the hub station are utilized effectively, and a data interface 15 which outputs a received packet data signal xe2x80x9cSexe2x80x9d and processes a transmitting packet data signal xe2x80x9cSfxe2x80x9d inputted from an external terminal (not shown).
Next, an operation of the conventional transmitting and receiving apparatus for satellite communications is explained. In case that a signal is received, first, a radio wave from the hub station is received at the antenna 11. This signal received at the antenna 11 is demodulated at the MODEM 13 via the transmitter/receiver 12. This demodulated data is inputted to the random access controller 14. The random access controller 14 controls addresses of the demodulated data, when the demodulated data has an address for this transmitting and receiving apparatus for satellite communications, the random access controller 14 outputs the received data as the received packet data signal xe2x80x9cSexe2x80x9d via the data interface 15 to the external terminal (not shown).
In case that a signal is transmitted, a transmitting packet data signal xe2x80x9cSfxe2x80x9d from the external terminal (not shown) is inputted to the data interface 15. The transmitting packet data signal xe2x80x9cSfxe2x80x9d is inputted to the random access controller 14 via the data interface 15. In case that plural transmitting data collide at the communications satellite and the transmitting data does not arrive at the hub station, and this transmitting and receiving apparatus for satellite communications can not receive an arrival confirmation signal for this own address from the hub station. In this case, a random access processing is performed at this random access controller 14 in order to retransmit the transmitting data. The transmitting packet data signal xe2x80x9cSfxe2x80x9d from the random access controller 14 is modulated at the MODEM 13 and is radiated from the antenna 11 via the transmitter/receiver 12 as a radio wave.
At this type conventional transmitting and receiving apparatus for satellite communications, the data interface 15 has a physical interface such as the number of connectors and also performs a logical interface processing data with the external terminals. That is, the data interface 15 consists of a microprocessor unit (MPU) and a memory, in order that the data interface 15 needs a protocol processing function. In case that a generally used MPU is used, there is a limit to the number of the physical interfaces connecting to the external terminals depending on the throughput of the generally used MPU. As a result, the throughput must be increased by a parallel processing used plural MPUs and bus lines, and dual port memories. In this case, the number of the physical interfaces can be increased, however there is a problem that the size of the apparatus becomes large.
Furthermore, in case that a large number of physical interfaces is required at the same ground station site, that is, a large number of packet data signals are transmitted and received to and from the hub station, a plurality of the transmitting and receiving apparatuses for satellite communications shown in FIG. 1 must be provided. In this case, there are problems that the size of the facilities becomes large and the investment becomes large, further the large area for installing the apparatuses is required.
In order to solve these problems, for example, Japanese Patent No. 2810063 describes a satellite communications system, Japanese Patent Application Laid-Open No. SHO 63-166329 discloses a transmission line control system, Japanese Patent Publication No. HEI 4-45020 discloses a forced transmission stopping system, and Japanese Patent No. 2526788 describes a transmitting and receiving apparatus for satellite communications in a ground station.
However, at the satellite communications system of Japanese Patent No. 2810063, the transmission line control system of Japanese Patent Application Laid-Open No. SHO 63-166329, and the forced transmission stopping system of Japanese Patent Publication No. HEI 4-45020, the size of processing is too large and the processing can not be performed by the generally used MPU. And when a large number of packet data signals are transmitted, a large size of physical interfaces are required, therefore there is a problem that it is difficult to realize the operation by using the generally used MPU. As a result, there is, a problem that it is difficult to increase or decrease the number of apparatuses corresponding to the amount of the transmitting data, for applying to VSATs, which are required to be carried and installed easily. Furthermore, at the transmitting and receiving apparatus for satellite communications in a ground station described in Japanese Patent No. 2526788, the size of the circuits can be reduced, however, when plural transmitting packet data signals are transmitted at the same time, it takes long time to retransmit the discarded transmitting packet data signals, therefore there is a problem that the transmission delay becomes large.
It is therefore an object of the present invention to provide a transmitting and receiving apparatus for satellite communications, which can make the transmission delay caused by retransmission minimize without increasing the sizes of signal processing and apparatus, and further with which the degree of freedom for alteration of total construction of apparatuses corresponding to the amount of the transmission data can be obtained, and which can be easily carried and installed.
According to a first aspect of the present invention for achieving the object mentioned above, there is provided a transmitting and receiving apparatus for satellite communications, in which packet data are transmitted and received by a random access processing Said transmitting and receiving apparatus for satellite communications provides plural random access control means; for applying a random access processing to a transmitting data from each of external terminals, provided for each of said external terminals, a collision detecting means for detecting collision of plural transmitting data transmitted from said plural random access control means, when said plural transmitting data are transmitted at the same time, a selecting means for selecting one of said plural transmitting data from said plural, random access control means, a transmitting means for converting said transmitting data selected at said selecting means to a signal to be transmitted, and an antenna in which said signal to be transmitted from said transmitting means is radiated as a radio wave. And said plural random access control means retransmit said transmitting data not selected at said selecting means by making the transmitting timing of said transmitting data not selected at said selecting means delay when the collision of said plural transmitting data is detected at said collision detecting means.
According to a second aspect of the present invention in the first aspect, said plural random access control means retransmits said transmitting data not selected without waiting for reception of an arrival confirmation signal for the transmitted data from a hub station, in case that said transmitting data not selected is retransmitted.
According to a third aspect of the present invention in the first aspect, said antenna is used for both transmitting and receiving data. And said transmitting and receiving apparatus for satellite communications further provides a receiving means for converting said signal received at said antenna to received data to which a signal processing can be applied. And said selecting means supplies said received data from said receiving means to said plural random access control means, and said plural random access control means output only the received data corresponding to its own address to said corresponding external terminal.
According to the present invention, when said collision detecting means detects the collision of plural transmitting data from said plural random access control means, said random access control means retransmits said transmitting data not transmitted caused by the collision without waiting for the reception of an arrival confirmation signal for the transmitted data from a hub station. Therefore, the transmission delay caused by the retransmission can be made to be minimum. Moreover, said antenna and a transmitter/receiver are used as a common for plural external terminals, with this, the sizes of signal processing and apparatuses are not increased greatly. Consequently, the degree of freedom for the construction corresponding to the amount of the transmitting data can be increased and further the apparatuses can be easily carried and installed.